it would be hard to say, considewring we dont know the full power of itachi (and we also dont know if that was the full power of his abilitys).. but then again we havnt seen the full powers of pain aswell, he was sick too. and almost everyfight we've seen from him, he has always had the odds against him and never went full force at the beginning. but from what we've seen so far i would have to say pain.
the reason is, is that itachis genjutsu (from what we know) can only tag one person (or body) at a time, and we can not compare jmans genjutsu because they were 2 different types of gen's (sound, and eye contact to itachi) sound goes everywere so its hard to not get caught in it,., but anyway, and its been shown that the susano'o isnt indestructible (but itachi has them damn mystical weapons)..
idk, but if pain starts off hard, then its over for any body. gedo mazou, chibaku tensei, or overwhelm him with summonings while reviving every one then its over for itachi even with susano'o (considering he cant stay in it very long).. pains chakra is on another level, so ill go for pain.. and im a bigger itachi fan so my pick isnt bias at all like most others.